The invention is directed to a printer means having an electrothermically operated printing head that includes a plurality of individual heating elements drivable in pulsed fashion, whereby an inking agent or a writing medium is locally heated in character dependent fashion via the heating elements in the writing mode and is transferred onto a recording medium by an aggregate status changed and is also directed to a method for the operation thereof.
Printer means of the species initially cited known either as thermotransfer printer means or as a bubble jet printer means. They are generally referred to as electrothermic or thermoelectric printers.
In thermotransfer printer means, an inking ribbon containing solid ink is locally heated in character-dependent fashion via a thermocomb having heating elements and the ink is thus melted pixel-by-pixel and is transferred onto a recording medium arranged behind the inking ribbon.
In bubble jet ink printer means as disclosed, for example, in German published application 30 12 946, a plurality of individual heating elements drivable in pulsed fashion are contained in ink channels in an ink printing head. These heating elements are flooded by a writing fluid and are locally heated in write mode. The heating elements generate local vapor bubbles in the writing fluid that effect the ejection of ink droplets out of the ink channels.
Heating elements of the electrothermically operated printers are usually composed of semiconductor resistor elements that are driven in pulsed fashion via a heating current. The writing speed obtainable with such printers is essentially limited by the degree of the residual heat of a writing event and by the elimination thereof. At high writing frequencies, the writing head heats until its function is no longer guaranteed. The basic temperature of the writing head dependent on environmental influences thereby has a significant influence.
In order to assure a reliable operation of thermoelectric printing heads, it was hitherto standard to design the level of the writing frequency to continuous operation of all writing or, respectively, heating elements and to adapt the heating duration of the heating elements to the most unfavorable operating conditions as well as printer tolerances.
Thus, German published application 36 12 469 discloses an electrothermically operated printer means wherein the operating frequency of the ink printing head is varied in accord with the temperature. The temperature of the ink printing head is thereby acquired via a temperature sensor attached in the head.
Such a temperature measurement is imprecise because it fundamentally covers only the average temperature of all heating elements but not the temperature behavior of an individual heating element. Further, such a temperature measurement has a great chronological lag compared to the heat emission of the individual heating elements. When, for example, an individual heating element is operated in continuous mode, then this quickly leads to a local overheating; the overall heat emission at the head, however, is low.
Further, TE-A33 00 395 discloses an apparatus for ejecting liquid droplets upon employment of thermal energy. In order to identify whether fluid is present at a nozzle, a conductive sensor element is arranged separately from the heating element of a nozzle at a distance therefrom.